DD2 has always been tiny - hovering between the 2nd and 10th percentile for most of her life. She had a check up today and is on the 10th percentile. She barely eats but is constantly on the go. DD2 has been seeing a paed over the last 15 odd months and he has no concerns about her weight, despite knowing of her incredibly poor diet, weight etc.

So long as your little man is active, alert etc I really wouldn't stress about it, especially if its a genetic thing in your family.